I have a 26 foot high tunnel with a tent stove. Frankly, I have found that composting raised beds do good job of heating it. The stove is for brief, extreme, temperature plunges. I also keep over 100 gallons of water in it.
"Good decisions come from experience. Experience comes from bad decisions." ... Mark Twain
I'm thinking solar electric panels would be too inefficient to heat a greenhouse. Solar thermal may be worthwhile but you'd need a lot of them. Better greenhouse design (insulation, passive solar) is probably where I'd start my efforts.
"Hundreds of years from now it will not matter what my bank account was, the sort of house I lived in or the type of car I drove... But the world may be different because I did something so bafflingly crazy that it becomes a tourist destination"